Venus Transit

July 16, 2004

On June 8, for the first time since 1882, the planet Venus came between the Earth and the Sun. The circumstances for these special transit events occur only every 122 years, and then in pairs separated by about 8 years. We will have an-other in 2012, but then not again until 2117 and 2125. The size of the planet Venus is about one arc minute, or about the size of a typical sunspot on the solar disk. Therefore as the planet moves across the Sun, we expect a dip in solar irradiance of about 0.1%. The SORCE TIM instrument routinely records the passage of sunspots with comparable dips in TSI. (The very largest sunspot groups late in October of 2003 produced a drop in TSI of more than 0.3%.) Observing the transit of Venus is well within the precision and accuracy capability of the SORCE TIM, and we anticipated that this astronomical event was an opportunity to test the instrument, and may provide some unique scientific data as well.

SORCE schedulers and scientists took advantage of the opportunity to capture this rare alignment by planning special experiments.